A cryogenic FBG sensor device is described in an article by T. Habisreuther et al., titled “ORMOCER Coated Fiber-Optic Bragg Grating Sensors at Cryogenic Temperatures”, IEEE Sensor Journal, Vol. 12, No. 1, pp. 13-16, 2012.
An FBG device comprises an optical fiber wherein a section of the fiber acts as an optical grating, wherein the index of refraction of the fiber varies periodically as a function of position along the axial direction of the fiber. The optical grating causes an optical wavelength dependence of the light reflection in the fiber, typically with a peak at a wavelength that corresponds to the optical period length of the periodic variation. An FBG sensor device makes use of the dependence of the optical period length on an external parameter, such as stress applied to the fiber, or its temperature.
The sensitivity of the optical period length in the optical fiber to external influence decreases with decreasing temperature. This limits the usefulness of FBG sensor devices for temperature measurement at cryogenic temperatures, e.g. for monitoring superconducting magnet systems, space applications, quantum computing/communication etc. Other fiber optic sensor devices, such as devices wherein the fiber is part of an interferometer or an optical frequency domain reflectometer suffer from similar problems.
Habisreuther et al discloses that use of an ORMOCER coating makes it possible to realize a temperature sensitivity of the period length of about 1 pm/K at a temperature of 40 K. However, the sensitivity below 20K is small.